A first for me - Frozen HX!

[stee6043]

I ran into an interesting situation yesterday. I'm 90% sure I froze my boiler HX with my air conditioning (froze on the outside of the HX, not inside). I ran the AC for several weeks last year and never had any issues. But yesterday I let the heat in the house get away with me and didn't turn the AC on until it had hit 86 in the house. To get the house down to 79 took 6-7 hours. Shortly before hitting my set temperature the air coming from the vents dramatically reduced. I went downstairs and everything was running but the fan sounded "odd". When I got closer my furnace was pushing a lot of air out of the small crack between the AC unit and the furnace. I was also feeling a fair amount of air coming out of the gap around my boiler HX (more than normal).

My only conclusion is that the extreme humidity combined with the extended run froze up my HX yesterday which prevented flow of cool air through the house. Anyone else ever done this? I was moderately concerned that freezing the water inside the HX would be extremely bad. But pressure was holding so I can only assume this didn't happen.

Am I the only one to have experienced this? Traffic is slow in the boiler room this time of year so I thought this might be an interesting discussion...

 

[DaveBP]

I know that a lot of refrigeration equipment uses cycle timers on the evaporation coils to stop occasionally and let the heat exchanger defrost. I think they have heaters in there along the coils to melt off the ice quickly. Not surprised that your coils could ice up.

 

[benjamin]

I'm 95% sure you didn't hurt anything. Is your heat exchanger (hot water/air coil?) between your blower and your AC A-coil (or whatever evaporator coil)? Unless you have a flat evaporator coil mounted directly to a flat water coil, it would be almost impossible to freeze the water inside, which would be very bad eventually, maybe not the first time. Evaporator coils freeze up all the time, much more often than many homeowners realize and many HVAC installers would admit. It does help keep them clean though. The blower will speed up when the airflow is blocked, so it will sound like there is less load, maybe a higher whine instead of a lugging.

 

[stee6043]

Thanks, Ben. My boiler HX is mounted 12-18" above my AC coil assembly, downstream. So it does get the coldest air but it's certainly not mounted directly to the coil. It would be a huge mess if I were to actually freeze and break that HX. But I too find it hard to believe it could get that cold. I'm just glad I didn't have a "real" problem with my furnace. My first thought as I walked downstairs last night involved $$$$$$'s flowing out of my wallet....

 

[taxidermist]

Thanks, Ben. My boiler HX is mounted 12-18" above my AC coil assembly, downstream. So it does get the coldest air but it's certainly not mounted directly to the coil. It would be a huge mess if I were to actually freeze and break that HX. But I too find it hard to believe it could get that cold. I'm just glad I didn't have a "real" problem with my furnace. My first thought as I walked downstairs last night involved $$$$$$'s flowing out of my wallet....

Check for dirty filter. And remember your AC runs faster fan speed than your heat so it will not sound the same. I ran my AC just like you yesterday and I had no problems.

Rob

 

[stee6043]

Well I pulled the cover off the AC unit last night after it stopped working again around 9PM. My A-Coil was a 100% solid block of ice. I did a little reading online and it sounds like a low refrigerant level could be the cause of my freezing problems. I'm going to have to break down and call a pro. My AC unit is only 7 years old so it seems like it shouldn't need a recharge yet....but who knows.

 

[RobC]

I use this time of year to clean out the entire, filter, blower, HX area. If you have a condensate pump that, the pipe to it and the discharge, needs a good cleaning, mine usually grows a nice crop of algae. They do make a biocide degreaser for cleaning coils but I use Simple Greeen and a good BACK FLUSH through the HX coils.
I needed to cut in access "doors" to be able to vacuum and wash so I got some left over storm door Plexiglas glass and used the metal HVAC tape to cover the doors. This way you can look into the guts of the unit during the year to see in that forbidden land, and to use for access in the future.
Cheers, Rob

 

[webie]

If I was a betting man I would say you are low on refrigerant . You may be able to nurse it along for a while by letting it cycle so it can thaw out . We use to have the same problem where i worked and after a summer of calling the HVAC guy every month and him looking for a leak and rechargeing it the compressor took a crap and after it was replaced the problem disapeared .

 

[RobC]

Fan Speed, I turned my fan speed to low because on high I wasn't able able remove enough humid air from the home. Some consolation is that HVAC fans are a are a high electricity consumer. Low uses substantially less KW's than high, the condenser will use the same, but you won't be comfortable in a cold wet home. You will need much colder setting like 72 instead of 75, for example and you'll feel like a clam.
Rob

 

[benjamin]

don't throw in the towel yet, unless you want the ac working and don't feel like messing around with it. it may be inadequate airflow causing it to freeze up, not a low charge. your poor blower has to move air through an A coil, hot water coil, possibly condensing furnace, possibly better than average air filter. not much you can do about those except the filter, don't tell anyone I said so, but the wet A coil is an excellent air filter, if you don't have cats etc. you could remove the air filter for the cooling season. How did the A coil look? it could be plugged with a layer of lint and mold, forcing the air around it instead of through it, or it could have too many leaks around it.

 

[stee6043]

The A-coil was actually very clean after I defrosted it. I did remove the filter last night but the more I read online the more I convinced myself it must be a refridgerant issue. I ran the same setup last year with no problems (including the boiler HX). When the unit is defrosted we get significant air movement both in and out.

I called in a pro this morning. He said it's likely in need of a recharge and he's going to stop out today to check it. He said a standard recharge runs $90. I didn't really want to spend the coin but in it's current condition it's not doing anyone any good. The efficiency must be horrible once it starts icing. That costs money too. We hit record highs this week (near 90) and I dare say my wife will kill me if I don't keep the house at or below 80!

Photo's of my ice blocks attached...

 

[RobC]

Here's a source for high efficiency / capacity air filters. I use them, good people, real good product. Pay attention to which side goes out when installing them. Coils are a pain to clean when NOT filled with crud, and it's amazing to see what comes out with good filters. Put dirt (no air filter in line) and water (condensate) and you have the recipe for cement in coils. The more wet dirt you have hanging around in there you can start to rot the coils, grow mold, loose efficiency......
http://www.pureairsystems.com/BioFilters.dept
Rob.

 

[benjamin]

I still don't know. I've still got a hunch it's airflow. It looks like I can see the fins on the coil so it's not that iced up. What did the underside of the coil look like?

I thought a starved coil would freeze unevenly. Any chance your blower is shutting off and back on?

I can't wait to hear what it was.

 

[stee6043]

I still don't know. I've still got a hunch it's airflow. It looks like I can see the fins on the coil so it's not that iced up. What did the underside of the coil look like?

I thought a starved coil would freeze unevenly. Any chance your blower is shutting off and back on?

I can't wait to hear what it was.

It was several pounds low on refridgerant. The guy told me when they freeze due to low refridgerant they do start to freeze at the bottom and move up the coil. I only opened the unit up after air flow had completely stopped so I can't confirm this. The fins were completely frozen over on both sides, the picture may not have shown this well.

After the recharge I ran it last night and it ran perfectly. It kept up much better and we had no freezing issues. All is well in the world. Another learning experience for me!

 

[taxidermist]

I still don't know. I've still got a hunch it's airflow. It looks like I can see the fins on the coil so it's not that iced up. What did the underside of the coil look like?

I thought a starved coil would freeze unevenly. Any chance your blower is shutting off and back on?

I can't wait to hear what it was.

It was several pounds low on refridgerant. The guy told me when they freeze due to low refridgerant they do start to freeze at the bottom and move up the coil. I only opened the unit up after air flow had completely stopped so I can't confirm this. The fins were completely frozen over on both sides, the picture may not have shown this well.

After the recharge I ran it last night and it ran perfectly. It kept up much better and we had no freezing issues. All is well in the world. Another learning experience for me!

Thanks for sharing learn something new everyday

 

[ihookem]

WAIT, were not done yet. I know an AC man that sais you have to put the heat coil below the ac coil or it will freeze. So, I call the AC/ hydronic heating man that plumbed up my EKO and said NO, that's not true, it won't happen to mine because he installed a high efficiency coil. The hgh efficiency coil has less resistance than a cheap one. He also stated a cheap coil might freeze up because it doesn't get as much air flow. Use a good hx coil I guess and don't worry about it . I seem to have noticed though that my AC does not seem to cool the house quite as fast as before I had the hx coil installed. It might be my imagination but my wife ( the queen of fussyness) says it doesn't cool quite as fast. I don't run AC much anyway though.